1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to a turbine blade with a spar and shell construction.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In a gas turbine engine, such as an aero engine used to power an aircraft or an industrial gas turbine engine used to produce electrical power, a turbine section includes a plurality of stages of rotor blades and stator vanes to extract the energy from the hot gas flow passing through. The engine efficiency can be improved by increasing the temperature of the hot gas flow entering the turbine. However, the inlet temperature is limited to the material properties of the first stage vanes and rotor blades. To improve the efficiency, a complex internal cooling circuits have also been proposed to provide impingement and film cooling to these airfoils in order to allow for a higher gas flow temperature.
A recent improvement in the high temperature resistant airfoils is the use of a spar and shell construction in which a shell having the shape of the airfoil is secured to a spar for support. The shell is typically made from a material that cannot be cast or forged like the nickel based super-alloys used to make turbine blades and vanes. The shell is fabricated from exotic high temperature materials such as Niobium or Molybdenum or their alloys in which the airfoil shape is formed by a well known electric discharge machining process (EDM) or a wire EDM process that can make a thin wall shell suitable for near wall impingement cooling in an airfoil. Because the turbine blade would be under high centrifugal forces during operation, the shell could even be made from a ceramic material because the spar would support the load, allowing for the shell to be exposed to the high temperature gas flow.
Turbine rotor disks also include blade attachment slots in which a root of the turbine blade having a fir-tree configuration is inserted to secure the blade to the rotor disk in the radial direction. The single piece cast nickel super-alloy turbine blade includes the root portion with the fir-tree configuration to fit within the disk slot. There is a need in the prior art for a spar and shell constructed blade to be capable of replacing the nickel super-alloy blade by using the attachment slot within the rotor disk to insert the spar and shell constructed blade.
The Prior Art U.S. Pat. No. 4,790,721 issued to Morris et al on Dec. 13, 1988 and entitled BLADE ASSEMBLY discloses a turbine blade with a metal core having a cap or blade tip, a metal liner functioning as a coolant containing surface, and a ceramic blade jacket secured between the blade tip of the metal core and the platform of the base having the fir-tree configuration. The metal core that holds the ceramic blade jacket (the shell) is secured to the fir-tree base by bonding. This construction is considered to be very weak in holding the blade together during operating speeds producing high centrifugal forces that tend to pull the spar away from the fir-tree root or base. Also, this construction does not permit removal and replacement of the shell component, which is known to be the life limiting part of the spar and shell constructed blade.
It is an object of the present invention to provide for a turbine blade of the spar and shell construction that can be inserted into the dove-tail slot of the rotor disk.
It is another object of the present invention to provide for a turbine blade having a spar and shell construction that can withstand the high centrifugal forces during operation of the engine.
It is another object of the present invention to provide for a turbine blade of the spar and shell construction with a fir-tree root in which the shell can be easily replaced.